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(Leipe, 1989). The contractile vacuole system 
exits to the exterior through a pore supported by 
helically disposed microtubules (Rodrigues de 
Santa Rosa, 1976). In some Holophrya (formerly 
Prorodon ) species, there is a set of collecting
canals extending anteriorly beneath the somatic 
cortex from the posterior contractile vacuole . These 
canals are associated with prekinetosomal micro-
tubules , which may provide structural support
(Hiller, 1993a). 
 13.4 Oral Structures 
 As the class name suggests, the prostomes are cili-
ates whose oral region is at the anterior end of the 
body, although it may be slightly subapical (Fig. 
13.2). The oral opening is typically permanent and 
flanked by oral dikinetids that border one side of 
the oral region in Plagiocampa (Foissner, 1978) 
or surround the entire cytopharyngeal complex 
(Hiller, 1991, 1993b; Huttenlauch, 1987). Brosse 
polykinetids are situated on the opposite side to the 
oral dikinetids in Plagiocampa (Foissner, 1978), 
almost enclosed by them in some Coleps species 
(Wilbert & Schmall, 1976), or completely enclosed 
by them in Balanion species. The brosse kinetids 
of Balanion may be reduced to two kinetosomes 
with reduced ciliation (Bardele, 1999; Jakobsen 
& Montagnes, 1999). The brosse in other genera 
can be more extensive, ranging from one up to 
five rows of kinetosomes embedded in the somatic 
infraciliature. In Holophrya (formerly Prorodon ) 
species, Hiller and Bardele (1988) have identified 
two major patterns: the aklitoloph pattern in which 
the somatic kineties do not terminate laterally 
on the brosse rows and the enklitoloph pattern in 
which the somatic kineties terminate on the right 
of the brosse rows (i.e., dexiotrop ), on the left of 
the brosse rows (i.e., aristerotrop ), and on both 
sides of the brosse (i.e., syntrop ). They speculated 
that these patterns might indicate four genera of 
 prorodontids . A brosse has also been identified in 
Cryptocaryon (Diggles, 1997). 
 Early ultrastructural research on prostomes sug-
gested that it was transverse microtubules arising 
from the oral dikinetids that surrounded the oral 
region (de Puytorac & Grain, 1972). Huttenlauch 
and Bardele (1987) demonstrated that these micro-
tubules were in fact postciliary microtubules and 
that the oral dikinetids rotated into place during 
stomatogenesis to give this unusual orientation 
(see below Division and Morphogenesis ). Both 
anterior and posterior kinetosomes of the oral 
dikinetids bear a postciliary ribbon and may also 
have one or two transverse microtubules (Hiller, 
13.4 Oral Structures 265
1993b). The postciliary microtubules appear to 
be used in several different ways among the pros-
tomes. In Holophrya (formerly Prorodon ) species, 
the oral postciliaries may extend anteriorly sup-
porting cortical ridges that cover the walls of the 
oral opening (Hiller, 1993b). In Balanion , Coleps , 
and Plagiocampa , oral palps , bearing a toxicyst ,
are placed internal to the oral dikinetids (e.g., 
Bardele, 1999; Fauré-Fremiet & André, 1965b). 
These palps are supported by microtubules, pre-
sumably derived from the postciliary ribbon of 
the anterior kinetosome of the oral dikinetids 
(Bardele, 1999). In a third group of species (e.g., 
Balanion , Bursellopsis , Urotricha ), the postciliary 
microtubules of the posterior kinetosome of the 
oral dikinetids extend around the perimeter of the oral 
opening in a counter-clockwise direction as viewed 
from outside the cell. They may overlap as many as 
12 other ribbons and appear to be joined together by 
intermicrotubule bridges, which may permit slid-
ing to dilate or close the oral opening (Hiller, 1991, 
1993b). The oral dikinetids typically sit atop two 
nematodesmata that are triangular to trapezoidal in 
cross-section. Each nematodesma extends into the 
cytoplasm and is joined by dense material deeper 
in the cytopharyngeal basket to the nematodesma 
arising from a neighboring oral dikinetid (Hiller, 
1993b; Lynn, 1985). Since postciliary ribbons sup-
port the oral cavity and line the cytopharynx, the 
 cytopharyngeal apparatus of prostomes must now 
be designated as a cyrtos – the rhabdos is now only 
found in the Class LITOSTOMATEA . 
 The oral ridges on the walls of the oral cavity 
have been observed in all prostomes , including 
Cryptocaryon (Colorni & Diamant, 1993; Diggles, 
1997). These ridges are covered by cortical alveoli 
down to the level of the cytostome (Hiller, 1993b; 
Hiller & Bardele, 1988; Huttenlauch, 1987; de 
Puytorac, 1964). There are typically two sets of 
microtubules in these ridges, one set composed of 
more microtubules than the other (Hiller, 1993b; 
Lynn, 1985). However, these can be reduced 
to 3 + 2 in Bursellopsis , very reminiscent of 
the oral ridges in oligohymenophoreans (Hiller, 
1991). These microtubules are presumably derived 
from the postciliary ribbons of the oral dikinetids 
(Huttenlauch & Bardele, 1987), and ultimately 
extend to line the cytopharynx where they may 
function to bring new food vacuole membrane 
to this region (Hiller, 1993b; Rodrigues de Santa 
Rosa, 1976). 
 The brosse or brush is considered here as an oral 
structure. As noted above, its kinetosomes can be 
within the circumoral dikinetids , opposite a paroral 
set of oral dikinetids or outside the oral perimeter 
entirely. In both colepids and prorodontids , the 
 brosse is composed of rows of dikinetids, whose 
axes are typically oriented perpendicularly to the 
longitudinal axis of the brosse rows. The anterior 
(or left) kinetosome bears a typically shorter, 
clavate-like cilium and may or may not bear a 
tangential transverse ribbon while the posterior (or 
right) kinetosome is non-ciliated and bears a postciliary
ribbon (Hiller, 1991, 1993b). 
 In species without oral palps , toxicysts are 
concentrated in the oral region, where they are 
presumably used to soften prey, like the fila-
mentous alga Spirogyra (Leipe, 1989). Similar 
to the long toxicysts of Dileptus , the toxicysts 
of Holophrya (formerly Prorodon ) demonstrate 
a combination of tubule evagination and tele-
scoping (Hausmann, 1978). Placus exhibits an 
unusual fosette or pit at the posterior end of its 
single brosse row. The brosse row extends into 
this pit in which there is a dense aggregation of 
 toxicysts (Fryd-Versavel, Iftode, & Dragesco, 1976; 
Grain et al., 1979). 
 13.5 Division and Morphogenesis 
 Prostomes typically divide while swimming freely. 
However, histophagous and parasitic prorodontids,
like Holophrya and Cryptocaryon , can form 
 reproduction or division cysts (Fig. 13.1) (Czapik, 
1965; Diggles, 1997; Hiller & Bardele, 1988). 
Even microphagous forms can form temporary 
 division cysts (Tannreuther, 1926). Hiller (1992) 
has reviewed the merotelokinetal stomatogenesis 
of prostomes and noted that since proliferation of 
kinetosomes occurs in a localized region of the cor-
tex, this region should be denominated ventral. The 
oral region becomes prostomatous or apical follow-
ing cytokinesis by allometric growth of somatic 
kineties that “push” the ventral oral region ante-
riorly (Fig. 13.5). The brush or brosse , therefore, 
cannot be considered a dorsal structure, although 
Hiller (1992) prefers not to consider it ventral. 
 As viewed from outside the cell, the circumoral 
ciliature forms by a clockwise migration of the cir-
cumoral dikinetids to form a partially or completely 
closed circle. In Coleps , the circumoral arises from 
one somatic kinety (Fig. 13.4; Huttenlauch & 
Bardele, 1987). The number of kineties involved in 
 circumoral dikinetid formation probably increases 
with increasing cell size : only two in Urotricha
(Muñoz, Téllez, & Fernández-Galiano, 1989), three 
to five in Bursellopsis (Hiller, 1992), and up to six 
in Holophrya (formerly Prorodon